Optical disc apparatus and discriminating method for kind of objective lenses

1. An optical disc apparatus comprising: a plurality of light sources emitting light beams having different wavelengths respectively; a plurality of objective lenses one of which is disposed selectively in a light path in response to an optical recording medium to condense on a recording surface of the optical recording medium a light beam emitted from one of the plurality of light sources, which is selected in accordance to the optical recording medium; a spherical aberration compensating element disposed between the plurality of light sources and the objective lens disposed in the light path to perform compensation of spherical aberration; a memory portion memorizing information; an amplitude obtaining portion obtaining S shaped waveform of focus error signal by emitting the light beam from one of the plurality of light sources and moving the objective lens disposed in the light path in an optical axis direction and obtaining amplitude from the S shaped waveform; and a discriminating portion changing controlled variable of the spherical aberration compensating element into a plurality of prescribed control values, obtaining, for each of the plurality of prescribed control values, the amplitude of S shaped waveform from the amplitude obtaining portion, and performing discrimination to determine whether one of the plurality of objective lenses is disposed in the light path based on the differences of changing rates of the amplitude of S shaped waveform changed from the controlled variable among the plurality of objective lenses.

2. The optical disc apparatus according to claim 1 , wherein the discriminating portion normalizing the amplitude of S shaped waveform obtained such that the amplitude of S shaped waveform obtained for one of the plurality of prescribed control values is a constant, calculating an inclination of approximated straight line showing relation between the controlled variable and the normalized amplitude of S shaped waveform, and discriminating whether one of the plurality of objective lenses is disposed in the light path by comparison between the inclination of approximated straight line and a threshold value memorized beforehand in the memory portion.

3. A discriminating method for an objective lens, discriminating the objective lens disposed in a light path of an optical pickup device including; a plurality of light sources emitting light beams having different wavelengths respectively; a plurality of objective lenses one of which is disposed selectively in the light path in response to kind of an optical recording medium to condense on a recording surface of the optical recording medium a light beam emitted from one of the plurality light sources, which is selected in accordance to the optical recording medium; and a spherical aberration compensating element disposed between the plurality of light sources and the objective lens disposed in the light path to perform compensation of spherical aberration, the discriminating method comprising: a first step changing controlled variable of the spherical aberration compensating element into a plurality of prescribed control values, and obtaining, for each of the plurality of prescribed control values, S shaped waveform of focus error signal by emitting the light beam from one of the plurality of light sources, moving the objective lens disposed in the light path in an optical axis direction and obtaining amplitude from the S shaped waveform; a second step normalizing the amplitude of S shaped waveform obtained such that the amplitude of S shaped waveform obtained for one of the plurality of prescribed control values is a constant; a third step calculating an inclination of approximated straight line showing relation between the controlled variable and the normalized amplitude of S shaped waveform; a fourth step deciding whether one of the plurality of objective lenses is disposed in the light path by comparison between the inclination of approximated straight line and a threshold value obtained beforehand.